The present invention relates to signal processing systems and more particularly to improved systems and techniques for providing plural frequency discrimination without the use of filter circuits.
Generally, in signal transmitting and receiving systems, such as radar systems, wherein it is desired to simultaneously transmit a plurality of carrier frequencies while discriminating between the received frequencies, the prior art has utilized transmitters and receivers which employ frequency filters to separate the carriers and define fixed frequency bands. Such systems, exemplified by fire control radar systems, may employ a CW carrier as the missile guidance microwave frequency and a pulse modulated carrier as the search and tracking frequency. The two carrier frequencies, which may be generated by employing a diplexer and CW reject filters, can be simultaneously transmitted through a single antenna as long as the frequency separation between the carriers is great enough to enable the receiving filters to reject the CW carrier while passing the pulsed carrier.
While such systems have had some success in providing for multiple frequency transmission and reception, the power losses incurred and the restrictions on frequency separation have severely limited their usefulness in a variety of situations. By way of example, such systems are incapable of providing the frequency agility that may be necessary in countermeasure environments since the filter requirements fix the frequencies to predefined bands during operation. In addition, the available frequency range is further limited by the wide frequency separation required for proper filtering in the transmitter and receiver. The filters, in turn, provide additional attenuation losses and matching problems that reduce the quality and power of the signals received in the receiver processing circuitry. Such limitations severely affect the range, accuracy, and environments in which the conventional systems can be readily employed.
As an alternative to the use of filters, other systems have attempted to utilize various configurations of multiplexing and switching circuitry to enable plural signal transmission. Such systems usually employ CW and pulsed signals alternately transmitted from the same antenna depending on the function desired at any given time. By way of example, the system as disclosed in U.S. Pat. No. 3,688,313 utilizes the CW signal to obtain initial target acquisition information relating to velocity and frequency, and thereafter switches to an angle tracking and search mode using pulsed signals for further processing of desired information. As can be seen, however, this system requires the transmission to be alternated between CW and pulsed and does not enable the simultaneous transmission of the plural signals from the same antenna.
Other systems, as exampled by U.S. Pat. Nos. 2,813,198; 2,909,774; 3,015,099; and 3,183,508; employ various configurations which enable the use of a single antenna for transmission and reception of one or plural signals. While each of the techniques attempt to solve particular problems with the described structure, there is still a continuing need for systems with improved versatility in the areas of frequency agility and high power protection for plural frequency operation.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above-known and similar techniques and to provide a system for enabling effective frequency discrimination and protection in a single antenna transmitting and receiving system.